The present invention relates generally to resonant free-piston type compressors and more particularly to such compressors which derive their motion from an electrodynamic oscillating motor.
In free-piston type devices to which the present invention relates the stroke is subject to certain variations since the piston is not guided in a fixed path by a mechanical means, such as a crank drive. This is an especially serious problem in free-piston resonant compressors where the piston derives its oscillating motion from an electrodynamic oscillating motor in that such compressors are only conditionally stable and instabilities in supply voltage or in the load will cause overstroking. That is, the piston stroke varies as a function of operational characteristics such as line voltage, characteristics of the compressed fluid, and the like.
In the past the variation of piston stroke with the compressor load has been a major problem. For example, as the compressor load increases there is a marked and undesirable decrease in the amplitude of the stroke. Also, if the compressor load decreases, there are no restraining forces acting upon the movable system and the amplitude of oscillation will increase until the piston strikes the cylinder head. One prior art approach to solving the overstroking problem was to provide for an enclosed volume of gas at the head of the cylinder which acted as a dash-pot to provide a gas cushion and prevent excessive overtravel. This is not an acceptable solution, however, since if the volume is of sufficient size to be effective, it adversely affects the volumetric efficiency of the compressor. While stroke control can be achieved with complicated electrical control means, this adds to the complexity and cost of the system and is not a satisfactory or acceptable solution either.
The desired stroke control is achieved in an entirely satisfactory manner in the foregoing referenced co-pending application by varying the stiffness of an enclosed volume of gas which acts as the resilient means of the resonant spring-mass system in accordance with the position of the piston.
It has been found desirable not only to provide for control of the maximum stroke limit but also to provide for a controlled variation of the stroke over varying limits. For example, it is desirable when the compressor is employed in a heat pump application to allow the stroke to change in order to properly function over a very wide range of load conditions. That is, in order to achieve a desired overall operating characteristic it may be necessary that the compressor operate with a shorter stroke at one compressor load condition and a longer stroke at another load condition. In one particular application for example, it is necessary that the compressor operate with a stroke of about 0.75 inch at one load condition and with a stroke of about 1.1 at another load condition. For stable operation it is necessary that although the stroke is allowed to vary over such limits it does so in a controlled manner and with a maximum overstroke limit under any operation condition.